Common rail fuel systems in which fuel is delivered by a delivery device into a fuel pressure storage device, the so-called rail, are believed to be available from the market. Multiple fuel injectors, which inject fuel directly into the assigned combustion chambers, are connected to the fuel pressure storage device. The pressure in the fuel pressure storage device is detected by a pressure sensor and regulated by a setting device to a variable setpoint pressure. The fuel injection quantity injected into a combustion chamber of the internal combustion engine depends, on the one hand, on the actual pressure in the fuel pressure storage device and, on the other hand, on the triggering time and the associated injection duration of the fuel injectors.
The signal supplied by the pressure sensor may be faulty due to damage, targeted manipulation, e.g., in the context of engine tuning or because of a pronounced drift, which results in the actual pressure prevailing in the fuel pressure storage device deviating more than intended from the setpoint pressure. The service life of components of the fuel system may be reduced in this way, the mixture-forming quality may decline and there may be deviations in the actual injection quantity from a setpoint injection quantity. It is therefore desirable to constantly monitor the signal of the pressure sensor for plausibility during operation of the internal combustion engine, e.g., as is increasingly being required by legislation in the case of diesel engines. However, installation of two or more redundant pressure sensors whose signals are compared with one another is associated with high costs.
German patent document DE 10 2008 044050 A1 discusses a method for operating an internal combustion engine in which a test injection quantity is initially established. The internal combustion engine is then operated at a first setpoint pressure and a first triggering time corresponding to this setpoint pressure and the test fuel injection quantity to be injected. A variable, which characterizes the operating state and is influenced by the actually injected fuel quantity, is detected in this process. The internal combustion engine is subsequently operated at a second setpoint pressure and a second triggering time corresponding to this setpoint pressure and to the test fuel injection quantity, and the variable characterizing the operating state is detected again. An action is executed when the two detected variables differ by more than a limiting value. Since there is a general nonlinear relationship between the fuel pressure and the injected fuel injection quantity at a given triggering time, it may be determined in this way whether the actual fuel pressure deviates inadmissibly from the setpoint pressure, e.g., due to a pressure sensor defect.
However, use of the method described here during ongoing operation of the internal combustion engine would require the fuel pressure to be set at least intermittently to setpoint values which are suboptimal for operation, which would have a negative influence on the service life of fuel system components, on the mixture-forming quality, on the fuel consumption and emissions.